Childhood asthma and obesity are significant public health problems. Most prospective studies suggest that obesity increases the risk of asthma. But, some authors did not found this association. In this study the association between asthma and body mass index (BMI) was investigated. This case-control study was conducted on 200 asthmatic children aged 6-15 years and 200 children without asthma. The criteria for asthma diagnosis and its classification were on the basis of National Asthma Education and Prevention Program (NAEPP). BMI of patients and controls were also measured and BMI grater than 85% and 95% were defined as overweight and obese respectively. The data was analyzed by SPSS software. The BMI among the asthmatic children (17.9 kg/m2) was higher than the BMI among the non-asthmatics (16.5 kg/m2), P=0.0001. This relationship was significant in both males and females. 18% of asthmatic children were classified as overweight and 13.5% of them were obese versus 7.5% and 6% respectively in non asthmatics (P=0.0001). However, there was no significant relationship between severity, duration of asthma, kind of medication and BMI in children with asthma (P>0.05). Result of this study showed that there is an association between asthma symptoms and obesity in children. Therefore, any attempts for weight control in asthmatic children might be beneficial.